1. Field of the Invention (Technical Field)
The present invention relates to absorption spectroscopy, particularly to cavity-enhanced absorption spectroscopy.
2. Description of Related Art
Note that the following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
The field of cavity-enhanced spectroscopy has been the focus of intense activity as a result of the high sensitivity that can be obtained from a compact and relatively simple apparatus. The apparatus includes a light source, a detector, and an optical cavity that contains the sample to be measured (typically a gas or liquid, but sometimes an optical component), together with some means of quantifying the light transmitted through the cavity.
Cavity enhancement refers to the increase in absorption signal (relative to a single pass measurement) when light passes through an optical cavity of two or more mirrors, typically formed by depositing dielectric coatings on their surfaces to achieve power reflectivity >99%. Resonant light from inside the cavity leaks out through the mirrors with a characteristic time scale known as the cavity lifetime. The spacing of the mirrors depends on the application but is typically in the range from about one millimeter to about one meter. By measuring a cavity-enhanced signal, a spectrum of the sample can be recorded, or the optical losses due to the sample can be estimated. These losses can be used to quantify the amount of sample in the cavity, for instance the concentration of methane in a sample of room air. High sensitivity to small optical losses is obtained in part because light that enters the cavity makes a large number of passes through the cavity before it leaks out through one on the mirrors to a detector. There are a large number of variations on the method. The field has been reviewed, e.g., in Gagliardi, G., et al., Cavity-Enhanced Spectroscopy and Sensing, Springer Series in Optical Sciences (Book 179) (2014), Paldus, B. A., et al., “An historical overview of cavity-enhanced methods”, Canadian Journal of Physics 83(10), 975-999 (2005), and Busch, K. W., et al., Cavity-ringdown spectroscopy: an ultratrace-absorption measurement technique (Vol. 720), American Chemical Society (1999).
A variety of detection methods can be used with continuous wave (CW) light sources. The simplest approach—“integrated cavity output spectroscopy”—measures the average optical power transmitted through the cavity. This approach has the advantage of simple implementation, but the disadvantage that the signal depends not just on losses in the cavity, but also on losses in the optical path outside the cavity and on variations in the efficiency of both the source and the detector.
In the “Ring-down” approach, the amplitude of the light source is modulated rapidly compared to the cavity lifetime, for instance using an acousto-optical modulator to deflect a laser, or by modulating the current injected into a semiconductor device such as a light emitting diode. Light shines on the cavity and after it has built up to a sufficient level, the beam is shut off and the decay of light from the cavity is recorded—the “cavity ring-down.” This approach has the advantage that the shape of the decay curve can be analyzed. In the simplest case, the shape is a single exponential decay, which is characterized by a single time constant, τ. A variety of effects can lead to multi-exponential decay, which may require a more sophisticated analysis to determine precisely the cavity losses. A disadvantage of the ring-down approach is that the recording rate has to be significantly faster than the cavity lifetime. This high recording rate becomes especially problematic when the cavity lifetime is short.
Phase Shift Cavity Enhanced Absorption Spectroscopy or Cavity Attenuated Phase Shift Spectroscopy, Herbelin, J. M. et al., “Sensitive measurement of photon lifetime and true reflectances in an optical cavity by a phase-shift method”, Applied Optics, 19(1), 144-147 (1980); Engel, G. S., et al., “Innovations in cavity enhanced laser absorption spectroscopy: Using in situ measurements to probe the mechanisms driving climate change”, In Earth Science Technology Conference, Laser Sensor Technologies (2003); Kebabian, P. L., et al., “Detection of nitrogen dioxide by cavity attenuated phase shift spectroscopy”, Analytical Chemistry 77(2), 724-728 (2005), measures losses in a cavity by measuring the phase shift of the modulation frequency of modulated light transmitted through the cavity. The amplitude of the light incident on the cavity is modulated. A fixed modulation frequency f is usually chosen to be close to 2π/τ to optimize sensitivity to changes in τ. Because the amplitude of the light incident on the cavity is modulated, the amplitude of the light transmitted through the cavity is also modulated, but the amplitude of the modulation and its phase change as a result of the time spent on average in the cavity. When the cavity exhibits simple exponential decay, the phase, θ, of the transmitted light is shifted by:tan(θ)=−2πfτ. 
Thus, a measurement of the cavity phase shift θ is equivalent to a measurement of ring down time T, and both can be related to the losses in the cavity and hence of the concentration of analytes in the cavity. U.S. Patent Publication No. 20120212731 to Loock extends this approach by describing a method for measuring the phase shift of a cavity at several modulation frequencies in order to account for multi-exponential decay waveforms in the cavity. This method also allows the use of two light sources, each modulated at its own frequency, for detecting simultaneously in more than one wavelength band. The modulation frequencies are chosen ahead of time such that they and their harmonics don't interfere. Once these modulation frequencies have been chosen, Loock measures the phase of the transmitted signal. All these phase shift cavity enhanced methods require the accurate measurement of a phase. Furthermore, all these methods require the choice of a modulation frequency that optimizes the sensitivity of the spectrometer. The sensitivity may degrade if large concentrations of an analyte are present, which would cause a significant change in τ such that the modulation frequency f was no longer about equal to 2π/τ. The calibration of the spectrometer depends on the calibration of the phase shift measurement. If the phase shift measurement is implemented by separate analog x and y demodulations, then gain errors between the x and y channels can introduce calibration errors. If the phase measurement is implemented by digital means, then digitization effects may limit the resolution with which the phase can be measured. For instance, the SR830 digital lock-in amplifier from Stanford Research has a phase resolution of 0.01 degrees. This is equivalent to a noise level of about 0.00017 of the detected light. This is one hundred times greater than the shot noise of a 100 nW optical signal in a 1 Hz bandwidth. Thus, detection with such a lock-in amplifier will not achieve the full theoretical precision. Furthermore, digital phase detection methods are difficult to apply at high frequencies associated with cavities that have short storage times.
The “NICE-OHMS” method is presented in Ye, J., et al., “Cavity-enhanced frequency modulation spectroscopy: advancing optical detection sensitivity and laser frequency stabilization”, Optoelectronics and High-Power Lasers & Applications (pp. 85-96), International Society for Optics and Photonics (1998) and Ye, J., et al., “Using FM Methods with Molecules in a High Finesse Cavity: A Demonstrated Path to <10-12 Absorption Sensitivity”, in ACS Symposium Series (Vol. 720, pp. 233-256), American Chemical Society (1999). The authors use modulation of the wavelength of the light source at a frequency that exactly matches the free spectral range of the cavity. The modulation frequency is therefore fixed. Power builds up in the cavity at the laser wavelength and at the sidebands that match cavity resonances. Under this condition, the amplitude of the light transmitted through the empty cavity is not modulated. When a wavelength-dependent absorption feature is present, it interacts more strongly with one of the sidebands, and this unbalances the transmitted power so that the light transmitted by the cavity is modulated. The information about the concentration of the absorbing species is carried by the amplitude of the modulation.
Many researchers have used modulation techniques to lock the wavelength or frequency of a laser to a particular cavity resonance. Typically, the wavelength of the light source is modulated by a small amount compared to the cavity width, and the light transmitted or reflected by the cavity is measured by lock-in demodulation. The wavelength of the laser or the length of the cavity is adjusted so that the cavity mode and the laser maintain an alignment in wavelength. In the work of Romanini, D., et al. “Optical—feedback cavity—enhanced absorption: a compact spectrometer for real-time measurement of atmospheric methane.” Applied Physics B 83.4, 659-667 (2006), optical feedback from the cavity to a laser caused the laser to lock to a particular cavity mode by adjusting the laser frequency. In neither of these cases is the information about cavity losses encoded as a variable modulation frequency.
A number of researchers have used information about the phase and amplitude of spectroscopic signals to improve the spectrometer. U.S. Pat. No. 7,805,980 to Kosterev describes a photoacoustic gas analysis spectrometer in which a relatively high modulation frequency for the light source is chosen so that the time lag associated with conversion of the absorbed optical energy into an acoustic wave results in significant phase shift. The information about the concentration of the analyte is contained in the amplitude of the acoustic signal, while the phase of the signal at some pre-determined frequency confirms the identity of the analyte. When two analytes absorb light of the same wavelength, it is still possible to distinguish the concentrations of the two by choosing a detection phase that is orthogonal to the interfering analyte. U.S. Patent Publication No. 20110214479 to Kachanov discloses a photoacoustic gas detection apparatus that includes lock-in detection by multiplying a signal by a sine and cosine reference function that have a constant phase relative to a modulation function. The frequency of the modulation function is chosen to match the resonant condition that enhances the signal from the acoustic cavity-microphone system. The concentration information is derived from the amplitude of the detected signal. All these photoacoustic approaches require the measurement of the amplitude of a signal from a microphone to determine the concentration of a species in the spectrometer.
Other arguably related references in the field include U.S. Patent Publication No. 20130083328 to Koulikov, U.S. Pat. No. 6,924,898 to Deck, and U.S. Pat. No. 7,301,639 to Kebabian.